1. Field of the Invention
This invention generally relates to mobile telephones and, more particularly, to a global positioning satellite (GPS) receiver incorporated into a mobile telephone for emergency location reporting during intermediate shadowing of the receiver.
2. Description of the Prior Art
The space-based global positioning system (GPS) implemented by the United States Department of Defense constitutes a convenient instrumentality for determining geographical position in real time. The GPS is a multiple satellite-based radio positioning system in which each satellite transmits data that allow precise measurement of the distance from selected ones of the GPS satellites to the antenna of a user's receiver so as to enable the user to compute position, velocity and time parameters through known triangulation techniques. The signals provided by the GPS can be received both globally and continuously.
The GPS is intended for use in a wide variety of applications, including space, air, sea and land vehicle navigation, precise positioning, time transfer, altitude referencing and surveying. A typical GPS receiver comprises a number of subsystems, including an antenna assembly, an RF (radio frequency) assembly, and a GPS processor assembly. The antenna assembly receives and amplifies the L-band GPS signal prior to insertion into the RF assembly. The RF assembly mixes the L-band GPS signal down to a convenient intermediate frequency (IF) signal. This signal is then fed to the GPS processor assembly where the PRN (pseudo random) code modulating the L-band signal is tracked through code-correlation at the receiver. This provides the processing gain needed to achieve a signal-to-noise (S/N) ratio sufficient for demodulating the navigation data and signal-transmission time stamp. The Doppler shift of the received L-band signal is also measured through a carrier tracking loop. The code correlation and carrier tracking functions can be performed using either analog or digital signal processing.
By differencing the signal transmission time with the time of reception, as determined by the clock of the receiver, the (pseudo) range between the receiver and the satellite being tracked may be determined. The pseudo-range includes both the true range to the satellite and the affected offset of the receiver clock from the GPS master time reference. The pseudo range and Doppler measurements (and navigation data) from four satellites are used to compute a three dimensional position and velocity fix, and to calibrate the receiver's clock offset and provide an indication of GPS time. Other approaches are possible, as described in co-pending patent application Ser. No. 08/924,478 filed Aug. 25, 1997, now U.S. Pat. No. 5,752,218, issued May 12, 1998, by Daniel D. Harrison for "Reduced Power GPS-Based System for Tracking Multiple Objects from a Central Location". General information on GPS may be obtained from the book by Tom Logsdon entitled The Navstar Global Positioning System, published by Van Nostrand Reinhold (1992).
GPS receivers are now being placed in mobile telephones so that the telephone location can be reported to a central location during an emergency. U.S. legislation has mandated that by the year 2000 mobile telephone position be provided for all 911 calls. If a GPS receiver is used to obtain the telephone location, a position fix is not possible when the telephone is shadowed from the GPS satellites. Shadowing may occur when the telephone is indoors.